"Look, the strength of Weblogs lies in the blogging process, the reading, thinking, writing, publishing, responding process. But to get students to do that effectively, we have to be able to let them own their own spaces by allowing them to write about their own passions and develop their own audiences."

I agree, but that isn't school anymore. Anyone can make a compelling case for blogging in helping students write better, explore their interests and develop networks, but it has nothing to do with curriculum, assignments, or assessment. The engaged kids who love this stuff will go do it anyway (around topics they like, using language and content that would never fly in school), and the majority will view blogs as odious assignments like all the others.